Xueqin Jiang

We propose a discrete-modulated coherent-state basis-encoding quantum key distribution (DMCS-BE-QKD) protocol, where the secret keys are encoded in the random choice of two measurement bases and it only needs simple binary sequence error correction. We analyze the secret key rate of DMCS-BE-QKD protocol under collective attacks in the linear Gaussian channel. The results show that DMCS-BE-QKD can greatly enhance the ability to tolerate the channel loss and excess noise compared to the original DMCS-CVQKD protocol. Finally, a proof-of-principle experiment is conducted under a 50.5 km optical fiber to verify the feasibility of DMCS-BE-QKD.

Bypassing the use of quantum coherent source and active modulations, passive-state-preparation (PSP) continuous-variable quantum key distribution (CVQKD) with thermal source provides a solution of high-speed on-chip modulators. However, the field experiment of free-space PSP CVQKD has still not been realized due to the lack of efficient excess noise suppression techniques via high-loss free-space channels. Here, we realize the PSP CVQKD field test over an urban free-space channel with record-breaking attenuation from -12.24 dB to -15.59 dB. Specifically, a novel scheme is proposed to reduce excess noise from PSP, and efficient quantum coherence detection alongside advanced digital signal processing algorithms is developed to achieve low-noise synchronized raw data acquisition. The secure keys are successfully generated, with statistical summation values of 0.85 kbps during the day and 1.52 kbps at night.

Quantum networks revolutionize the way of information transmission and are an essential step in building a quantum internet. Generally, the information capacity per user-channel in a quantum network drastically decreases with the increase of network capacity, making it difficultly scale to large-user scenarios. To break this limit, we propose a network capacity-independent quantum network (NCI-QN) that maintains constant information capacity per user-channel regardless of network scale, overcoming the scalability bottleneck in conventional quantum networks. The architecture employs a multi-mode time-frequency framework, with theoretical analysis extending PLOB and Holevo bounds to network scenarios to establish capacity independence. Experimentally, we demonstrate a 19-user NCI-QN using optical frequency combs in quantum key distribution, achieving a record 8.75 Gbps composable finite-size secure key rate.